Oil & Gas Wells
To produce oil or gas from a reservoir, a well is drilled into a subterranean formation, which may be the reservoir or adjacent to the reservoir. Typically, a wellbore of a well must be drilled hundreds or thousands of feet into the earth to reach a hydrocarbon-bearing formation.
Generally, well services include a wide variety of operations that may be performed in oil, gas, geothermal, or water wells, such as drilling, cementing, completion, and intervention. Well services are designed to facilitate or enhance the production of desirable fluids such as oil or gas from or through a subterranean formation. A well service usually involves introducing a well fluid into a well.
Drilling is the process of drilling the wellbore. After a portion of the wellbore is drilled, sections of steel pipe, referred to as casing, which are slightly smaller in diameter than the borehole, are placed in at least the uppermost portions of the wellbore. The casing provides structural integrity to the newly drilled borehole.
Cementing is a common well operation. For example, hydraulic cement compositions can be used in cementing operations in which a string of pipe, such as casing or liner, is cemented in a wellbore. The cement stabilizes the pipe in the wellbore and prevents undesirable migration of fluids along the annulus between the wellbore and the outside of the casing or liner from one zone along the wellbore to the next. Where the wellbore penetrates into a hydrocarbon-bearing zone of a subterranean formation, the casing can later be perforated to allow fluid communication between the zone and the wellbore. The cemented casing also enables subsequent or remedial separation or isolation of one or more production zones of the wellbore by using downhole tools, such as packers or plugs, or by using other techniques, such as forming sand plugs or placing cement in the perforations. Hydraulic cement compositions can also be utilized in intervention operations, such as in plugging highly permeable zones, or fractures in zones, that may be producing too much water, plugging cracks or holes in pipe strings, and the like.
Cementing and Hydraulic Cement Compositions
In a cementing operation, a hydraulic cement, water, and any other components are mixed to form a hydraulic cement composition in fluid form. The hydraulic cement composition is pumped as a fluid (typically in the form of suspension or slurry) into a desired location in the wellbore. For example, in cementing a casing or liner, the hydraulic cement composition is pumped into the annular space between the exterior surfaces of a pipe string and the borehole (that is, the wall of the wellbore). The hydraulic cement composition should be a fluid for a sufficient time before setting to allow for pumping the composition into the wellbore and for placement in a desired downhole location in the well. The cement composition is allowed time to set in the annular space, thereby forming an annular sheath of hardened, substantially impermeable cement. The hardened cement supports and positions the pipe string in the wellbore and fills the annular space between the exterior surfaces of the pipe string and the borehole of the wellbore.
Spacer Fluids
Effective and complete removal of the drilling mud is required for a successful cement job. Spacer fluids are used to displace the drilling fluid from the well before cementing operation. The drilling fluid can be either water-based or oil-based system.
In the case of oil-based fluids, it is important to displace them completely from well, otherwise they will contaminate the cement slurry, which can eventually lead to issues such as incompatibility, poor bonding as well as suppression of compressive strength development. The presence oil layer over the casing may affect the bonding between the casing and cement and lead to formation of micro channels.
A surfactant is used in a spacer to enhance the compatibility between the spacer and oil-based drilling fluid. A surfactant also helps to change the interface between the mud and spacer from an oil-external emulsion to a water-external. In the past, a surfactant package comprising DSS-A (oil soluble), DSS-B (water soluble), and SEM-8 (water soluble) has been used extensively. It is desirable to use fewer surfactants in order to minimize the costs and associated expenses, such as transportation. In addition, there is a need for an environment friendly, salt-tolerant surfactant composition for spacer fluids.
Inorganic Salts in Spacer Fluids
Spacer fluids are often formed with water, seawater, or, for various reasons, inorganic salts such as NaCl or CaCl2 may be added. It is important that a surfactant for a spacer fluid be compatible for use with seawater or having other inorganic salts dissolved in the water. Not all surfactants are compatible for use with dissolved salts.